Molecular gas in NUclei of GAlaxies (NUGA)^*

XI. A complete gravity torque map of NGC 4579: new clues to bar evolution

¹ Observatorio Astronómico Nacional (OAN)-Observatorio de Madrid, Alfonso XII 3, 28014 Madrid, Spain e-mail: [s.gburillo;s.fernandez]@oan.es
² Observatoire de Paris, LERMA, 61 Av. de l'Observatoire, 75014 Paris, France e-mail: [francoise.combes;frederic.boone]@obspm.fr
³ Istituto di Radioastronomia/CNR, Sez. Firenze, Largo Enrico Fermi 5, 50125 Firenze, Italy e-mail: hunt@arcetri.astro.it
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany e-mail: haan@mpia.de;schinner@mpia-hd.mpg.de
⁵ Center for Astrophysics, SMA project, 60 Garden Street, MS 78 Cambridge, MA-02138, USA e-mail: mkrips@cfa.harvard.edu
⁶ Instituto de Astrofísica de Andalucía (CSIC), Apdo 3004, 18080 Granada, Spain e-mail: isabel@iaa.es

Received: 18 July 2008
Accepted: 14 January 2009

Abstract

In this paper we create a complete gravity torque map of the disk of the LINER/Seyfert 1.9 galaxy NGC 4579. We quantify the efficiency of angular momentum transport and search for signatures of secular evolution in the fueling process from r ~ 15 kpc down to the inner r ~ 50 pc around the active galactic nucleus (AGN). We use both the 1-0 and 2-1 line maps of ¹²CO obtained with the Plateau de Bure Interferometer (PdBI) as part of the NUclei of Galaxies-(NUGA)-project. The CO(1-0) and CO(2-1) PdBI maps at 2.0″  1.3″ and 1.0″  0.6″ resolution, respectively, include the short spacing correction provided by IRAM-30 m data. We derive the stellar potential from an NIR (K band) wide field image of the galaxy. The K-band image, which reveals a large-scale stellar bar and a weak nuclear oval, together with a high-resolution Hi map of NGC 4579 obtained with the Very Large Array (VLA), allow us to extend the gravity torque analysis to the outer r ≥ 2 kpc disk. 
The bulk of the gas response traced by the new CO PdBI+30 m maps of NGC 4579 presented here follows the expected gas flow pattern induced by the bar potential in the presence of two inner Lindblad resonances (ILR) at r ~ 500 pc and r ~ 1.3 kpc. The morphology of the outer disk traced by Hi suggests that the neutral gas is currently piling up in a pseudo-ring formed by two winding spiral arms that are morphologically decoupled from the bar structure. The pseudo-ring is located inside the bar corotation (r_CR ~ 6 kpc) and close to the predicted position of the ultra harmonic resonance (UHR) (r_UHR ~ 3.8 kpc). The derived gravity torque budget in NGC 4579 shows that inward gas flow is occurring on different spatial scales in the disk. In the outer disk, the decoupling of the spiral allows the gas to efficiently populate the UHR region, and thus produce net gas inflow on intermediate scales. The corotation barrier seems to be overcome by secular evolution processes. The gas in the inner disk (r ≤ 2 kpc) is efficiently funneled by gravity torques down to r ~ 300 pc. Closer to the AGN (r < 200 pc), gas feels negative torques due to the combined action of the large-scale bar and the inner oval. The two  modes act in concert to produce net gas inflow down to r ~ 50 pc, providing clear smoking gun evidence of inward gas transport on short dynamical timescales (~1-3 rotation periods).